1. Field of the Invention
The present invention relates to a stacked photovoltaic device and more particularly to a stacked photovoltaic device including a plurality of power generating units.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2000-58892 discloses a stacked photovoltaic device having a stacked structure in which a first power generating unit including a photoelectric conversion layer and a second power generating unit including a photoelectric conversion layer are stacked in sequence. This stacked photovoltaic device includes a thin film polycrystalline silicon layer or a microcrystalline silicon layer which acts as the photoelectric conversion layer of the first power generating unit, and an amorphous silicon layer which acts as the photoelectric conversion layer of the second power generating unit. Incidentally, output characteristics of the entire conventional stacked photovoltaic device mentioned above are determined according to a balance between the respective characteristics of the first power generating unit and the second power generating unit.
However, the conventional stacked photovoltaic device mentioned above has a disadvantage that a decreasing rate of characteristics of the second power generating unit alone becomes large, because the amorphous silicon layer that acts as the photoelectric conversion layer of the second power generating unit has a high rate of photodegradation. This disadvantage leads to another disadvantage of increasing the rate of photodegradation in the output characteristics of the entire stacked photovoltaic device including the first power generating unit and the second power generating unit. This results in a problem that a long-term use of the stacked photovoltaic device causes an increase in a range of variation in the output characteristics of the stacked photovoltaic device.